The present invention relates to a new and improved method of transmitting and receiving electrical speech or voice signals which are transmitted in ciphered or coded form, wherein at the transmitter side or part there are formed from the speech signal to be transmitted, by frequency analysis, signal components or parameter signals which, in intervals or sections, contain frequency spectrum coefficients, voiced/unvoiced information coefficients and fundamental sound pitch coefficients, these parameter signals are coded or enciphered, the coded parameter signals are transformed into a transmission signal and the latter is transmitted via a transmission channel, and further, wherein at the receiver side or part there is again obtained from the received transmitted signal the coded parameter signals and such are decoded or deciphered, and from the thus obtained decoded parameter signals there is produced by synthesis a speech signal similar to the original speech signal.
In heretofore known systems the coded signal components or parameter signals are transformed by means of a modulator device into a transmission signal which can be transmitted via a voice channel. This transmission signal consists of frequency modulated, phase modulated or otherwise modulated, sequentially transmitted pulses, the transmission rate amounting to, for instance, 1200, 2400 or 4800 bits/sec.
At the receiver end the received transmission signal, with the heretofore known installations, is demodulated by a demodulator device in order to again obtain the coded signal components or parameter signals.
A state-of-the-art installation which functions in accordance with the above-described technique is the so-called "Vocoder". This installation comprises a signal analysis device equipped with a multiplicity of band filters, this system serving to obtain the frequency spectrum coefficients. From the low frequency portions there is determined in a fundamental sound pitch detector the fundamental sound pitch coefficient of voiced sounds and from the energy relationship between the high and low frequencies there is determined at the voice detector the voiced/unvoiced information coefficient. In the signal synthesis device there is likewise present a multiplicity of band filters, the passband damping of which can be modulated by the frequency spectrum coefficients. At the output of such band filters there is obtained the synthesized speech when there is introduced at the input of the signal synthesis device for voiced sounds spike pulses in cycle with the fundamental sound pitch and for voiceless sounds a noise signal.
There are also known physical manifestations of such installations which possess, instead of the band filters, active filters, digital filters or transmission networks which can be modulated.
The heretofore known systems are associated with different drawbacks. For the conversion of the coded signal components or parameter signals appearing in the transmission signal they require the above-mentioned modulation-and demodulation devices, so-called MODEM units and additionally for the operation of such devices also parallel/series converters and series/parallel converters. Hence, there is required a considerable expenditure in such devices and converters.
The high rate of the series infed information during the transmission of, for instance, 4800 bits/sec. requires short pulse lengths of about 0.2 ms, rendering the transmission difficult at narrow band transmission channels. Due to the short pulse length there is increased the difficulty of attaining the synchronization which is important during ciphering. A further drawback of the heretofore known installations is the faulty quality of the speech or voice produced by synthesis, and such is attributable to the imperfect construction of the signal synthesis device. Finally, the known installations are relatively complicated in construction and design and are not readily suitable for realizing a uniform construction with highly integrated, digital circuit components, and moreover saving of circuit components through the use of sequential operating steps is only possible to a limited extent.